Generally this invention relates to the field of plasma-based thin film processing. Specifically, the invention focuses upon DC magnetron sputtering to coat a compact disc ("CD") and the like where precise edging is desired.
The sputtering process in general is well known in the art. It was apparently first reported in 1852 by Sir William Robert Groves. In 1921 Joseph John Thompson initially named the process "spluttering"; later the "1" was dropped and the name for the process became "sputtering". This term is now used to describe a process whereby atoms of a material are mechanically freed from the surface through a momentum transfer. The atoms then drift and eventually come in contact with a surface and cause a thin film on or interact with that surface or substrate. Although the sputtering process has been known for some time, in recent years applications of the process have grown dramatically and have been subject to refinement and development. To a significant degree, the increase and focus on the sputtering process has been due to the growth of the semi-conductor industry which has increased the focus on thin film processes and has made available more sophisticated equipment to practice the process.
To understand the interesting nature of the problem addressed by the present invention, it is necessary to generally understand magnetron sputtering. An excellent discussion of this process is contained in the text book Glow Discharge Processes by Brian Chapman published in 1980 by John Wiley & Sons, Inc., which is hereby incorporated by reference. As discussed with respect to the present invention, magnetron sputtering is a process whereby the surface of an item is coated. In basic form, the coating system involves an AC or DC power supply which, acting together with a magnetic field, ionizes a gas. The ions of this ionized gas or plasma then accelerate towards a target due to the presence of a DC potential created by the power supply. The target contains the material that will become the coating. When the ions strike the target, atoms of the target are released through momentum transfer. These atoms then drift free and some eventually contact a substrate and become a coating on that substrate. This substrate is often, but not necessarily, electrically disconnected from the power supply. As mentioned earlier, the sputtering process has been refined to allow tighter process control. In order to achieve this, it has been desirable to utilize switch-mode power supplies for a variety of reasons. These power supplies are fast acting in that they can react within nanoseconds to changing conditions such as that of the plasma involved. They are also power supplies which store low amounts of energy so that should an undesirable discharge occur, the amount of energy released to the process will be small enough so as to not significantly impact the process. Switch-mode power supplies are often also physically smaller than other types of designs.
Magnetron sputtering has a great variety of applications whether deposition or etching. Without limiting the application of the present invention, the present invention is discussed in the context of its utilization to coat a plastic or polycarbonate substrate with aluminum such as is accomplished in creating a compact disc. As a visual inspection of any compact disc will show, it is desirable to terminate the aluminum coating at some distance from the edge of the plastic substrate. This termination should be sharp and precise. This is accomplished by placing a mask over some portion of the substrate so areas are not coated. Unfortunately, for reasons not fully understood, small irregularities frequently occur. These irregularities are colloquially referred to as "mousebites". While in this particular application, the mousebites do not impact the performance of the coated item, they do represent a visual imperfection and so have been the focus of a number of attempts by those skilled in the art to avoid their occurrence. The problem, however, has proven to be enigmatic in that the solutions do not seem to conform to the present understanding of the nature of the problem. First, it appears from a microscopic examination of the irregularities that they are the result of local arcing and resultant damage of the thin coating. It has been discovered, however, that the utilization of higher energy storage power supplies (such as an SCR power supply) often completely eliminates the problem. This is paradoxical for two reasons. First, as mentioned earlier, in most applications the substrate is electrically disconnected from the power supply involved. By the very nature of the sputtering process, it would not be expected that the amount of energy storage in the power supply would have any impact on the existence or non-existence of mousebites. Second, even if one were to hypothesize some connection with the power supply, one would expect that the less energy stored there the less damage an arc would cause to the disc. The solution of using a high-stored-energy power supply, unfortunately, also eliminates the possibility of rapidly controlling the process as is possible through the use of a switch-mode power supply.
As a result of the limitations of the prior solution, almost universally, the solution implemented by those skilled in the art is to space the mask at some distance from the substrate. This can eliminate the local arcing that seems to be the direct cause of the mousebite phenomenon. Unfortunately, this solution results in a less focused edge and so it is not entirely satisfactory.
The present invention overcomes the need to accept either of the limitations of the prior solutions. It is an electrically-based solution involving a switch-mode power supply. Admittedly the present invention represents a solution to the problem which continues the enigmatic nature of the problem. As a result it can be said that it represents a non-obvious solution because even now the exact nature of the problem is not fully understood by those skilled in the art. Since the present invention represents a relatively simple solution, it underscores that the attempts by those skilled in the art to overcome the difficulties posed by the problem are a result of the difficulty in understanding the problem itself. Certainly, it can be said that those skilled in the art actually taught away from the direction taken by the present invention through their preconceptions concerning the nature of the problem. Thus, the result--complete elimination of the mousebite problem--is a totally unexpected result. Surprisingly, even though the problem of mousebites has existed for some time, and even though the implementing arts were easily available, the solution of the present invention fulfills the need through a very simple and straightforward technique.